A 47-year-old man believed to have been in contact with the suspected knifeman who killed three people at a church in the French city of Nice has been detained for questioning, a judicial source said Friday.

The man was detained late Thursday after the attack in Nice's Notre-Dame basilica by a 21-year-old Tunisian who arrived in France on October 9.

France's anti-terror prosecutor Jean-Francois Ricard said the attacker, identified as Brahim Aouissaoui, had a copy of the Koran, two phones and three knives when he entered the church in the centre of the Mediterranean city at around 8:30 am.

He slit the throats of a 60-year-old woman and a 55-year-old man who worked at the church, and stabbed a 44-year-old woman who managed to flee but later died of her wounds.

The victims were "people targeted for the sole reason that they were present in this church at that moment," Ricard said at a press conference late Thursday.

President Emmanuel Macron called it an "Islamist terrorist attack," and the government has placed its terror alert at maximum ahead of the Catholic holiday of All Saints Day on Sunday.

Macron is holding an emergency meeting over the attack with top ministers Friday.

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